1. Field of the Invention
The invention is related to the field of shaped explosive charges used for oil well perforating. More specifically, the invention is a high-explosive shaped charge having an improved liner retention feature, and a method for making the improved high-explosive shaped charge.
2. Description of the Related Art
High-explosive shaped charges are used for, among other purposes, perforating steel casing cemented into wellbores drilled through the earth for extracting oil and gas. Shaped charges known in the art typically include a quantity of powdered high explosive, such as those known by trade names RDX or HMX, inserted into a housing. Some shaped charges can include an acceleration explosive disposed on one side of the housing for increasing the probability of detonation of the high explosive when a detonating signal is applied to the outside of the housing. The detonating signal is usually conducted through a detonating cord placed in contact with the side of the housing opposite to the intended detonation direction. The detonating cord is itself typically filled with high explosive similar to that used in the shaped charge and can be initiated by a blasting cap, exploding bridge wire or similar detonator. A liner is positioned in contact with the high explosive, opposite to the acceleration explosive, inside the housing. The liner is generally conically shaped and can be composed of powdered metal, such as copper and lead, formed by compression under extremely high pressure so as to behave substantially as a solid. See for example U.S. Pat. No. 4,794,990 issued to Riggs which describes the process of compressing the liner material into a finished liner.
When the high explosive is detonated it generates extremely high pressures and temperatures which cause the liner to discharge from the other side of the housing in a predetermined pattern, which is typically in the shape of a very narrow cone or "jet". The jet moves away from the housing at very high velocity, thereby generating the perforation in the wellbore by means of the kinetic energy of the jet. The force of the jet typically can penetrate a 1/2 inch thick steel casing inserted into the wellbore and cement surrounding the casing before penetrating several inches of earth formation.
As is known in the art, the performance of the shaped charge can be dependent on the shape of the liner, the position of the liner within the housing, and uniformity of contact between the high explosive and the liner. The performance of any individual shaped charge depends on the liner and the high explosive remaining in their correct relative positions even after handling and transportation of the shaped charge to the wellbore. The shock and vibration of transport, as well as thermal cycling during storage, sometimes cause mispositioning of the liner, the explosive, or both.
Several methods are known in the art to prevent shifting or settling of the explosive and to prevent movement of the liner. Referring to FIG. 1, which shows a typical shaped charge known in the art, a bonding agent 6 can be applied to the surface of the liner 4 which is placed in contact with the high explosive powder 8, in order to improve cohesion between the liner 4 and the explosive 8. Bonding agents can be difficult and expensive to apply to the liner 4, and can substantially increase the cost of making a shaped charge.
It is also known in the art to apply an adhesive, as shown at 10, to the discharge side of the liner 4 on its circumference, in order to bond the liner 4 to the inside wall of the housing 2. The adhesive 10 prevents movement of the liner 4 relative to the housing 2. The adhesive used for bonding the liner 4 to the housing 2 typically is of a type which can be cured by exposure to ultraviolet light. Use of the adhesive known in the art to bond the liner 4 to the housing 2 can be difficult and expensive, particularly because use of adhesive requires the additional step of removing or "dusting off" excess high explosive powder 8 which may be present on the part of surface of the housing 2 to which the adhesive is to be bonded.
U.S. Pat. No. 5,237,929 issued to Ekholm suggests interference fit of a retainer ring to prevent movement of the liner. The retainer ring shown in the Ekholm '929 patent includes protrusions for shaping the discharged "jet" into a special pattern, but the protrusions do not affect the liner retention aspect of the retainer ring. A difficulty in using any type of interference fit device to retain the liner is that thermal expansion can cause the retainer ring to become loose and allow the liner to move. Thermal expansion is a particular issue with oil well perforating charges because the temperature in a wellbore can easily exceed 300.degree. F. at the intended perforating depth. Interference fit liner retaining rings have therefore proven unsuitable for use in oil well perforating charges.
U.S. Pat. No. 5,351,622 issued to Ekholm discloses a resilient retainer to prevent movement of the liner. The resilient retainer described by Ekholm has several drawbacks. The housing requires a groove in order to better restrain the resilient retainer from axial movement. Providing a groove can increase the overall cost and complexity of the shaped charge. Further, for safety reasons, any groove in the housing would have to be thoroughly cleaned before inserting the resilient retainer to avoid accidental detonation of any explosive which might be deposited in the groove during manufacture of the shaped charge. The Ekholm '622 patent in fact describes the shaped charge as preferably being filled with high explosive through an opening in the rear of the housing (the side opposite the intended discharge) after insertion of the assembled liner and resilient retainer into the groove in the housing. Such "back-loaded" shaped charges are more difficult and expensive to make than the "front loaded" charges (those loaded through the discharge side) more commonly used in oil well perforating, primarily because the necessary opening in the rear of the housing would have to be plugged in a separate operation, requiring some type of plug to close the opening. The resilient retainer described in the Ekholm '622 patent prevents the liner from moving laterally by the spring force exerted by the retainer ring, but the retainer ring does not provide any spring force in a direction parallel to the discharge of the jet. It is possible for the high explosive in a shaped charge made according to the Ekholm '622 patent to settle and shift during transport and movement into a wellbore, unless the explosive is tightly packed against the liner. Back-loading the shaped charge as described in the Ekholm '622 patent would make packing the high explosive difficult because the small opening in the rear of the housing makes it impracticable to insert a suitable ram or packing device into the housing which could make contact with the high explosive across the entire area bounded by the liner.
Another type of retainer for the liner in a shaped charge is shown in U.S. Pat. No. 4,798,145 issued to McVeagh. The retainer shown in this patent comprises a spring washer which axially compresses the liner against the explosive. The spring washer disclosed by McVeagh has several drawbacks. First, the spring washer is fairly long, even when fully compressed against the liner. This length means that the shaped charge housing has to extend axially outward from the open end of the liner enough to provide a seat for gripping teeth on the outer end of the spring. According to the McVeagh '145 patent, the spring washer is relatively long so that a "bowed in" portion (shown at numeral 9 in FIG. 5 of the McVeagh '145 patent) will not interfere with the discharge from the detonating shaped charge. The required length of the spring washer and the housing makes it difficult to place the shaped charge in close contact with its target, as is frequently done in oil well perforating, for example. Second, the spring washer exerts lateral force on the circumference of the liner in an inward direction. McVeagh states that the liner and retaining spring can substantially avoid effects of thermal contraction by axially compressing the liner against the explosive, but FIG. 6 in the McVeagh '145 patent clearly shows a gap (numeral 27) between the liner and explosive when the shaped charge is subjected to low temperature. Inward lateral force only makes the problem of thermal contraction worse, as the spring's inward force combines with the thermal contraction.